Phosphorus and nitrogen are nutrients which can cause eutrophication and algal bloom, if they are discharged to sensitive waters in large amounts. On the other hand, phosphorus is essential to all living organisms. It is obtained almost exclusively from mined phosphate rock (USGS, 2010; van Vuuren et al., 2010, see the References). Predictions vary, but it is commonly believed that 35-100% of the current estimated phosphorus reserves will be depleted by the end of the century (Dery and Anderson, 2007, van Vuuren et al., 210). The crucial role of phosphate for food production makes phosphorus recovery a high priority.
The conventional method to remove phosphorus and nitrogen is to adopt a biological nitrogen and phosphorus removal process in the biological sewage treatment works, such as the 5-stage Bardenpho Process, as shown in FIG. 1. In this process, nitrogen is removed through autotrophic nitrification followed by heterotrophic denitrification. The nitrification step requires a large amount of oxygen and a large reactor, because the nitrifiers are slow growing bacteria. Denitrification is then conducted by converting organic carbon to carbon dioxide and nitrate to nitrogen gas. Phosphorus removal is conducted through luxury intake by means of the addition of an anaerobic zone at the beginning of the sewage treatment plant. Phosphorus harvesting was then conducted by controlled struvite (Magnesium Ammonium Phosphate, or MAP) precipitation from the anaerobic sludge digester supernatant or by sludge incineration. Overall, the biological nutrient removal process requires a large land footprint to cater for the slow growth bacteria.
Urine represents roughly 1% of the bulk sewage liquid volume (Maurer et al., 2006) but represents approximately 80% of the total nitrogen load and 50-80% of the total phosphorus load in the bulk sewage volume (Fittschen and Hahn, 1998). Since this phosphorus stream comprises roughly 5% of mined phosphorus loads (Cordell et al., 2009; van Vuuren e al., 2010), source separation of urine provides a significant opportunity for phosphorus recovery.
Urine separation has been studied and implemented in an urban environment since the 1990s in some European countries, such as Sweden and Denmark (Hanöus et al., 1997; Jönsson et al., 1997) with over 3000 systems installed in Sweden by 1999 (Hellström and Johansson, 1999). Urine separation is accomplished through a specially designed NoMix toilet bowl with a small compartment at the front designed to collect urine and by urinals in male toilets. They are currently available on the market and are made by a number of manufacturers. Studies in Northern Europe have shown that NoMix technology is generally well accepted by users in numerous surveys (Berndtsson, 2006; Lienert et al., 2007). Although scaling and blockage has been an issue in the early development of the system, these problems are no longer of major concern (Jönsson, 2001).
Phosphorus and nitrogen from urine can be reused through direct application of urine onto agricultural lands. However, direct discharge of urine to agricultural lands causes human hygienic concerns. Moreover, as there is a high possibility of urine being contaminated by endocrine disrupters which originated from drugs taken by humans, direct reuse of urine is not truly welcomed for growing edible crops.
Phosphorus recovery can be achieved through addition of magnesium salts to urine to facilitate precipitation of magnesium ammonium phosphate (MAP) (Maurer et al., 2006). This is a solid fertilizer free from micro-pollutants and the majority of heavy metals (Ronteltap et al., 2007). This can therefore be a safer phosphorus recovery method. However, this process suffers from the need for addition of expensive magnesium salts. Thus, a low-cost alternative is needed, especially for developing countries. One readily available and low-cost source of magnesium would be seawater, which contains 1.29 g/L of magnesium.
Research has been reported regarding wastewater treatment using seawater, e.g., phosphorus recovery from digester supernatant with seawater (Kumarshiro et al., 2001; Lee et al., 2003) and phosphorus recovery from urine with bittern (Etter, 2009). However, these processes involve relatively complex chemical or biochemical processes, microorganisms and/or source of materials and relatively high cost.
U.S. Pat. No. 4,228,003 to Makino discloses removal of phosphates from wastewater by way of coagulation and sedimentation of phosphates by adding seawater. This process requires adjustment of pH and a specific ratio of phosphate-containing wastewater to seawater. However, this process requires removal of phosphorus directly from municipal sewage wastewater and no urine separation is required.
In addition, US 2008/0308505 to Jansen et al. discloses a system and a process for removal of phosphorous and ammonia from aqueous streams, U.S. Pat. No. 7,005,072 Bowers et al. discloses a method for removing phosphorus from waste lagoon effluent, U.S. Pat. No. 7,722,768 to Abma et al. discloses a process for the simultaneous removal of BOD and phosphate from waste water, U.S. Pat. No. 4,911,843 to Hunniford et al. discloses a process for removal of dissolved hydrogen sulfide and reduction of sewage BOD in sewer or other waste systems, and U.S. Pat. No. 7,404,897 to Bailey Jr. et al. discloses a method for nitrogen removal and treatment of digester reject water in wastewater using bioaugmentation. However, no report or disclosure so far has combined urine and seawater or seawater toilet flushing.